The invention relates to an illumination unit comprising an electrodeless low-pressure discharge lamp and a supply device, which lamp has a discharge vessel enclosing in a gastight manner a discharge space provided with an ionizable filling, and which lamp in addition has a coil provided with a winding around a sintered core of polycrystalline ferrite material, which winding is connected to the supply device, the coil being capable of inducing a high-frequency magnetic field which maintains a discharge in the discharge space.
The invention also relates to an electrodeless low-pressure discharge lamp and to a coil suitable for use in such an illumination unit.
An illumination unit of the kind described in the opening paragraph is known from U.S. Pat. No. 4,710,678. In a nominal operating condition, the high-frequency magnetic field caused by the coil generates an electric current in the discharge space which keeps the discharge going. The known illumination unit is ignited in that a resonant circuit, which includes the coil, is brought into resonance. A comparatively high voltage arises across the coil during this. This voltage is used to generate an electric field by means of one or several conductors adjacent the discharge vessel, for example the winding of the coil, thus initiating the discharge. The losses occurring in the illumination unit during nominal operation are mainly due to the thermal losses occurring in the discharge space, as are the losses in a conventional low-pressure discharge lamp in which the discharge is maintained by electrodes. These losses amount to approximately 60% of the power consumed by the illumination unit. Losses specific to the electrodeless lamp, i.e. the losses in the electric circuit and the losses in the coil, are found to be small compared therewith, approximately 11% and approximately 5%, respectively, of the power consumed. The efficacy of the electrodeless lamp, accordingly, is of the same order of magnitude as that of a conventional low-pressure discharge lamp. Such an illumination unit has the advantage that the lamp can have a comparatively long operational life, because electrodes in the discharge space are unnecessary.
It was found, however, that the life of the illumination unit can be limited by that of the supply device.